A problem which is commonly encountered in automobiles is that, for reasons which may be inadvertent, a battery loses power due to drainage of current from it, and the main object of this invention is to provide a cut-off circuit which will be effective in prevention of the battery drainage.
This problem has been recognized and various attempts have been made to provide a satisfactory voltage monitoring circuit, and in some of those attempts a low voltage cut-out circuit has also been provided.
In the German patent DE 3742312 in the name of Wemhoner, there is described a circuit wherein a comparator compared a voltage reference taken from a battery with a fixed voltage source, and when the comparator changed state upon the battery voltage diminishing, a relay was actuated to remove inessential loads from essential loads. The division of inessential and essential loads within a vehicle wiring system is also used in other prior art, but involves considerable wiring and is inconvenient for after-market installation.
In the Australian PCT application AU89/00249, and the U.S. Pat. Nos. 4,137,557 and 4,088,940, the division between essential and inessential loads was also made in the wiring. The relay coil was driven by an amplifier. However, there was no time delay mechanism, and no override. Furthermore, it was possible that the system itself could be responsible for drainage of energy from a battery. None of the aforesaid patent specifications taught the use of any current limiting device, although the German patent specification 3742312 utilised latching relays. These are not looked upon with favor by the applicant because of cost and power rating.
The General Motors Corporation Patent U.S. Pat. No. 4,493,001 was essentially an original equipment device because of the large number of connections required in the motor vehicle. It did not however utilise ignition sensing bypass circuits. However, it had a manual override switch which was incorporated in the ignition switch system of the vehicle, and also an anti-theft switch.
Finally, in the U.S. Pat. No. 4,218,717 in the name of Shuster, a system was disclosed which utilised a current threshold relay. The system monitored the current for excessive drain for excessive period of time, and it triggered upon excessive current flow. It was not sensitive to small current drain as for example maybe caused by the parking lights of the vehicle, but it did have a time delay circuit. The circuit was bypassed by the ignition switch.